The present invention is generally directed to methods and devices for reading and decoding single width bar codes. More particularly, the present invention is particularly directed to the decoding of scanned bar code patterns which may be presented either as dark bars on a light background or as light bars on a dark background. The present invention also includes a preprocessor for removing "glitches" that may be present as a result of the scanning operation. Additionally, the present method and apparatus is particularly amenable to incorporation of check digit symbols, redundancy and repeated measurements all of which contribute to the overall robustness of the decoding.
The utilization of bar codes has become a very convenient phenomena. Many people are already familiar with the universal product code (UPC) typically employed at supermarket checkout counters not only for purposes of speeding data entry but also for inventory control and the creation of detailed receipts. This is but one example of the utility found in bar code marking systems. The UPC code is however a multi-width bar code and therefore, as a result, reading and decoding operations are more complicated than necessary.
Typically, a bar code pattern is provided on a product by marking with dark bars on a white background. This background may be provided as part of a separate adhesive label or the bar code may be printed as part of the packaging of the box containing various forms of merchandise. However, in certain circumstances it is desirable to provide bar code markings directly on materials like glass, aluminum, plastic, silicon and the like. This is particularly important in those cases when the object being marked will be subjected to process steps which would be inimical to such things as paper labels. In these instances, bar code patterns may be present in the form of light bars on a relatively dark background. Accordingly, it is therefore seen to be desirable to have a mechanism for reading bar code patterns having either of these formats, that is, independently of the nature of the relationship between the background and the foreground in terms of lightness and darkness. It would be desirable to have this operation performed automatically and certainly have it performed in a manner in which the recognition rate is extremely high. It would also be desirable to achieve these goals without the utilization of expensive equipment.
The desirability of reading black on white bar codes and white on black bar codes is evidenced in U.S. Pat. No. 4,656,345, issued Apr. 7, 1987 to Yukuo Kurimoto which is assigned to Tokyo Electric Co., Ltd. and which was filed in the U.S. on Sep. 11, 1985. However, the bar code reader described therein depends upon the manual intervention of the user to operate a switch which in one position inverts the output from a wave shaping circuit. Such a mechanism is naturally disadvantageous in that it requires prior knowledge of the nature of the bar code in order for the system to function. Applicants' invention overcomes this and other problems.